The "formula" to calculate the amount of EXP required for each level changes throughout the leveling process.

However, it only change during some "important checkpoints"; namely Level 15, Level 20, Level 35, Level 50, and Level 70.

As such, the algorithm to calculate the amount of EXP required can be described as:

    + Level 1 - 15: Every level require exactly 20 EXP.

    + Level 16 - 20: Every level require 100 EXP more than the last level.

    + Level 21 - 35: Every level require 120 EXP more than the last level.

    + Level 36 - 50: Every level require 140 EXP more than the last level.

    + Level 51 - 70: Every level require (lvl * 5) EXP [rounded to the next ten] more than the last level.

        E.g: Level 51 require (51 * 5 = 255, rounded to 260) more EXP compared to level 50.

    + Level 71 or higher:

        * There's a hidden variable that represent the difference between the EXP required for the current level in comparison to the previous level.

        * This hidden variable starts at 720 at level 71 and is increased by {value1} every level, and {value20} every 20 levels.

        * Additionally, every 20 levels, {value1} is increased by 10, while {value20} is increased by 200. This increment happen before the calculation for the hidden variable.

        * At level 71, hidden variable = 720, {value1} = 10, {value20} = 710.

    Pseudocode:

exp[70] := 10520;
val1 := 10;
val20 := 710;
inc := 720;

For (lvl := 71 to 300){
	exp[lvl] := exp[lvl - 1] + inc;
	If (lvl - 70) is divisible by 20 {
		val1 := val1 + 10;
		val20 := val20 + 200;
		inc := inc + val20;
	}
	inc := inc + val1;
}

Note that due to the natural limitation of time and resources, the formula is not guaranteed to be correct for all levels.

However, it's still correct so far at level 250+, so it's pretty safe to say that the algorithm is correct and won't be changed in the forseeable future.